1. Field of the Invention
The present invention generally relates to chemical vapor deposition (CVD) reactor systems and, more particularly, to inverted CVD reactor chambers that use a vertical reactant gas flow to permit the growth of semiconductor, insulator, and metal layers having uniform physical and electrical characteristics.
2. Description of the Prior Art
CVD reactor systems may be used for the deposition of homoepitaxial, heteroepitaxial, and polycrystalline layers of semiconductors, insulators, and metals (hereinafter generally referred to as epitaxial and dielectric layers) on the surface of semiconductor substrates. The layers are deposited as a result of a pyrolytic decomposition reaction of reactant gas phase compounds transported into close proximity with the substrate by a carrier gas, typically hydrogen. The substrate is at a temperature above that necessary to initiate the thermal decomposition of the reactant gas compounds. Thus for example, a silicon epitaxial layer can be grown on the surface of a silicon substrate by the pyrolytic decomposition of silane. A gallium arsenide epitaxial layer may be epitaxially grown on a gallium arsenide substrate by the decomposition of trimethyl gallium and arsine. Dielectric layers of, for example, oxide and nitride compounds can be grown through the pyrolytic decomposition of appropriate gasous phase reactant compounds in oxygen and nitrogen ambients. Doping of epitaxial layers can be accomplished by including additional reactant gas phase compounds in the reactant gas mixture. Upon decomposition of the additional compounds, the various doping species are deposited on the growth surface of the epitaxial layer and are incorporated therein.
In the design of CVD reactor systems, substantial emphasis is placed on enabling the growth of physically uniform, high electrical quality layers. This is particularly desirable in the growth of epitaxial semiconductor layers, since uniformly thin epitaxial layers grown over large substrate surface areas are required for the subsequent fabrication of LSI and VLSI integrated circuits. The ability to obtain epitaxial layers having specific carrier concentrations and dopant concentration profiles across several sequentially grown epitaxial layers is equally of importance. Further, the ability to grow both epitaxial and dielectric layers having extremely low contaminant and defect densities is important in obtaining the desired optimum electrical characteristics.
In CVD reactor systems, two related phenomena are recognized as contributing to unpredictable variations in the carrier concentration and doping density profile of CVD grown epitaxial layers and for the introduction of contaminants into both epitaxial and dielectric CVD grown layers. The first phenomenon is generally referred to as autodoping. This phenomenon is typically encountered in the growth of an effectively or near intrinsic semiconductor epitaxial layer, such as low impurity silicon or high resistivity gallium arsenide. Characteristically, the resultant epitaxial layers are found to have a significant impurity concentration; the impurities effectively acting as dopants and, thereby, preventing the accurately reproducible growth of the desired epitaxial layers. These impurities are conventionally thought to arise from contaminants present in the reactant gases and from the exposed surfaces of the structures necessarily present within the reactor system. Naturally, these same contaminants produce defects and unpredictable electrical quality variations in both epitaxial and dielectric layers grown by CVD.
The second phenomenon is generally referred to as the memory effect. This phenomenon is principally encountered during the growth of a semiconductor epitaxial layer. Unlike the autodoping phenomenon, the impurity sources giving rise to the memory effect phenomenon are known and well appreciated. The memory effect impurities are essentially the dopants intentionally utilized to dope previously grown epitaxial layers and the semiconductor substrate itself. In particular, a residual amount of the dopant transport compound utilized during a previous CVD growth may be effectively delayed or temporarily trapped within the CVD reactor system and, therefore, only reaches the epitaxial growth surface during a subsequent epitaxial layer growth. The delay or trapping of the dopant transport compound may be due to the presence of "dead spaces" within the reactor system, wherein the gaseous dopant compound languishes and only slowly diffuses back into the main flow of reactant gases moving toward the substrate.
A similar delay is introduced when a portion of the gaseous dopant transport compound is allowed to condense onto any of the inner surfaces of the reactor system. The dopant transport compound evaporates at some time thereafter and re-enters the main reactant gas flow.
Another impurity source giving rise to the memory effect phenomenon is created by the improper deposition of a dopant during an epitaxial growth onto an inner surface of the reactor chamber and not, as intended, onto the growth surface of the epitaxial layer. In a subsequent epitaxial growth, given that the improper deposition is onto a heated surface, some portion of the dopant species will evaporate and eventually become incorporated into the growing epitaxial layer. Naturally, the carrier concentration and doping profile will be unpredictably affected due to the unpredictable rate of evaporation.
Finally, the last recognized impurity sources giving rise to the memory effect phenomenon are the previously grown epitaxial layers and the substrate. Since the substrate is directly heated, along with any existing epitaxial layers during the growth of a succeeding epitaxial layer, a significant amount of the dopants will out-diffuse from these layers and become incorporated in the growing epitaxial layer.
Although there are a number of different impurity sources giving rise to the memory effect, the result in each case is the same. The impurities, acting as dopants, either partially compensate and decrease or directly and accumulatively increase the carrier concentration of the epitaxial layer being grown. Further, the dopant concentration profile across both the junction to the underlying epitaxial layer and within the growing epitaxial layer is unpredictable due to the equally unpredictable quantity and rate of arrival of the impurity dopant at the growth surface of the epitaxial layer. Consequently, the ability to accurately grow either abrupt or controllably graded homojunctions and heterojunctions, as well as simple uniformly doped epitaxial layers, is severly restricted.
The failure to achieve high physical uniformity, both in terms of thickness and composition, is another well recognized problem in the CVD growth of epitaxial layers. Non-uniform layer thicknesses directly result from the failure to uniformly deposit the various constituent components of the growing epitaxial layer evenly over the entire epitaxial growth surface. Likewise, non-uniformities in the composition of the grown epitaxial layer arise from the failure to uniformly deposit the appropriate proportions of each of the constituent components of the epitaxial material to achieve the desired stoichiometric composition.
Numerous reactor designs have been developed to specifically correct the physical non-uniformity problem. These designs treat the problem as principally arising from the depletion, through decomposition, of the reactant gases present through the length of the reactor chamber (U.S. Pat. Nos. 4,279,947 and 3,922,467), the varying distance of different areas of the epitaxial growth surface from the reactant gas inlet (U.S. Pat. Nos. 4,062,318 and 3,633,537), and convection currents generated in the reactive gases immediately adjacent the heated substrate as a result of the substantial thermal gradient necessary to induce the decomposition of the gases (U.S. Pat. No. 3,916,822). The device disclosed in this last patent provides for the creation of a convection-current free zone immediately adjacent an invertly mounted semiconductor substrate. An inlet manifold provides a continuing supply of reactant gases to a diffusing radiation shield. The reactant gases pass through the radiation shield and diffuse through the convection-current free zone into close proximity with the substrate surface. Following decomposition, the remaining vapor-phase reaction products then diffuse back through the convection-current free zone and the radiation shield to exit downwardly from the reaction chamber. The thickness variation reportedly achieved by the disclosed device is 4% over the surface area of a conventional 1.5 inch diameter semiconductor wafer. Though this is apparently the minimium thickness variation achieved in the entire prior art, it is still nearly twice the thickness variation that is estimated to be tolerable in the fabrication of LSI and VLSI integrated circuits prepared utilizing CVD epitaxial layer growth. The composition variation in epitaxial layers grown utilizing the disclosed device are not reported. The best composition variation reported in the prior art is apparently on the order of .+-.10% over a surface area of 15 cm.sup.2.